Steels are alloys of iron and carbon, widely used in construction and other applications because of their high tensile strengths and low costs. Carbon, other elements, and inclusions within iron act as hardening agents that prevent the movement of dislocations that otherwise occur in the crystal lattices of iron atoms.

The carbon in typical steel alloys may contribute up to 2.1% of its weight. Varying the amount of alloying elements, their formation in the steel either as solute elements, or as precipitated phases, retards the movement of those dislocations that make iron so ductile and weak, and thus controls qualities such as the hardness, ductility, and tensile strength of the resulting steel. Steel's strength compared to pure iron is only possible at the expense of ductility, of which iron has an excess.

Further refinements in the process, such as basic oxygen steelmaking (BOS), largely replaced earlier methods by further lowering the cost of production and increasing the quality of the metal. Today, steel is one of the most common materials in the world, with more than 1.3 billion tons produced annually. It is a major component in buildings, infrastructure, tools, ships, automobiles, machines, appliances, and weapons. Modern steel is generally identified by various grades defined by assorted standards organizations.

Alloys with a higher than 2.1% carbon content, depending on other element content and possibly on processing, are known as cast iron. Cast iron is not malleable even when hot, but it can be formed by casting as it has a lower melting point than steel and good castability properties.[1] Steel is also distinguishable from wrought iron (now largely obsolete), which may contain a small amount of carbon but large amounts of slag. Note that the percentages of carbon and other elements quoted are on a weight basis.

Iron-carbon phase diagram, showing the conditions necessary to form different phases

Iron is commonly found in the Earth's crust in the form of an ore, usually an iron oxide, such as magnetite, hematite etc. Iron is extracted from iron ore by removing the oxygen through combination with a preferred chemical partner such as carbon that is lost to the atmosphere as carbon dioxide. This process, known as smelting, was first applied to metals with lower melting points, such as tin, which melts at approximately 250 °C (482 °F) and copper, which melts at approximately 1,100 °C (2,010 °F). In comparison, cast iron melts at approximately 1,375 °C (2,507 °F).[2] Small quantities of iron were smelted in ancient times, in the solid state, by heating the ore buried in a charcoal fire and welding the metal together with a hammer, squeezing out the impurities. With care, the carbon content could be controlled by moving it around in the fire.

All of these temperatures could be reached with ancient methods that have been used since the Bronze Age. Since the oxidation rate of iron increases rapidly beyond 800 °C (1,470 °F), it is important that smelting take place in a low-oxygen environment. Unlike copper and tin, liquid or solid iron dissolves carbon quite readily. Smelting results in an alloy (pig iron) that contains too much carbon to be called steel.[2] The excess carbon and other impurities are removed in a subsequent step.

Other materials are often added to the iron/carbon mixture to produce steel with desired properties. Nickel and manganese in steel add to its tensile strength and make the austenite form of the iron-carbon solution more stable, chromium increases hardness and melting temperature, and vanadium also increases hardness while making it less prone to metal fatigue.[3]

To inhibit corrosion, at least 11% chromium is added to steel so that a hard oxide forms on the metal surface; this is known as stainless steel. Tungsten interferes with the formation of cementite, allowing martensite to preferentially form at slower quench rates, resulting in high speed steel. On the other hand, sulfur, nitrogen, and phosphorus make steel more brittle, so these commonly found elements must be removed from the steel melt during processing.[3]

The density of steel varies based on the alloying constituents but usually ranges between 7,750 and 8,050 kg/m3 (484 and 503 lb/cu ft), or 7.75 and 8.05 g/cm3 (4.48 and 4.65 oz/cu in).[4]

Even in a narrow range of concentrations of mixtures of carbon and iron that make a steel, a number of different metallurgical structures, with very different properties can form. Understanding such properties is essential to making quality steel. At room temperature, the most stable form of pure iron is the body-centered cubic (BCC) structure called ferrite or α-iron. It is a fairly soft metal that can dissolve only a small concentration of carbon, no more than 0.005% at 0 °C (32 °F) and 0.021 wt% at 723 °C (1,333 °F). At 910°C pure iron transforms into a face-centered cubic (FCC) structure, called austenite or γ-iron. The FCC structure of austenite can dissolve considerably more carbon, as much as 2.1%[5] (38 times that of ferrite) carbon at 1,148 °C (2,098 °F), which reflects the upper carbon content of steel, beyond which is cast iron.[6]

When steels with less than 0.8% carbon (known as a hypoeutectoid steel), are cooled, the austenitic phase (FCC) of the mixture attempts to revert to the ferrite phase (BCC). The carbon no longer fits within the FCC structure, resulting in an excess of carbon. One way for carbon to leave the austenite is for it to precipitate out of solution as cementite, leaving behind a surrounding phase of BCC iron that is low enough in carbon to take the form of ferrite, resulting in a ferrite matrix with cementite inclusions. Cementite is a hard and brittle intermetallic compound with the chemical formula of Fe3C. At the eutectoid, 0.8% carbon, the cooled structure takes the form of pearlite, named for its resemblance to mother of pearl. On a larger scale, it appears as a lamellar structure of ferrite and cementite. For steels that have more than 0.8% carbon, the cooled structure takes the form of pearlite and cementite.[7]

Perhaps the most important polymorphic form of steel is martensite, a metastable phase that is significantly stronger than other steel phases. When the steel is in an austenitic phase and then quenched rapidly, it forms into martensite, as the atoms "freeze" in place when the cell structure changes from FCC to a distorted form of BCC as the atoms do not have time enough to migrate and form the cementite compound. Depending on the carbon content, the martensitic phase takes different forms. Below approximately 0.2% carbon, it takes an α ferrite BCC crystal form, but at higher carbon content it takes a body-centered tetragonal (BCT) structure. There is no thermal activation energy for the transformation from austenite to martensite. Moreover, there is no compositional change so the atoms generally retain their same neighbors.[8]

Martensite has a lower density than does austenite, so that the transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally take the form of compression on the crystals of martensite and tension on the remaining ferrite, with a fair amount of shear on both constituents. If quenching is done improperly, the internal stresses can cause a part to shatter as it cools. At the very least, they cause internal work hardening and other microscopic imperfections. It is common for quench cracks to form when steel is water quenched, although they may not always be visible.[9]

There are many types of heat treating processes available to steel. The most common are annealing, quenching, and tempering. Annealing is the process of heating the steel to a sufficiently high temperature to soften it. This process goes through three phases: recovery, recrystallization, and grain growth. The temperature required to anneal steel depends on the type of annealing to be achieved and the constituents of the alloy.[10]

Quenching and tempering first involves heating the steel to the austenite phase then quenching it in water or oil. This rapid cooling results in a hard but brittle martensitic structure.[8] The steel is then tempered, which is just a specialized type of annealing, to reduce brittleness. In this application the annealing (tempering) process transforms some of the martensite into cementite, or spheroidite and hence reduces the internal stresses and defects. The result is a more ductile and fracture-resistant steel.[11]

When iron is smelted from its ore, it contains more carbon than is desirable. To become steel, it must be reprocessed to reduce the carbon to the correct amount, at which point other elements can be added. In modern facilities, this liquid is then continuously cast into long slabs or cast into ingots. Approximately 96% of steel is continuously cast, while only 4% is produced as ingots.[12]

The ingots are then heated in a soaking pit and hot rolled into slabs, blooms, or billets. Slabs are hot or cold rolled into sheet metal or plates. Billets are hot or cold rolled into bars, rods, and wire. Blooms are hot or cold rolled into structural steel, such as I-beams and rails. In modern steel mills these processes often occur in one assembly line, with ore coming in and finished steel coming out.[13] Sometimes after a steel's final rolling it is heat treated for strength, however this is relatively rare.[14]

South Indian and Mediterranean sources including Alexander the Great (3rd c. BC) recount the presentation and export to the Greeks of 100 talents of South Indian steel. The reputation of Seric iron of South India (wootz steel) amongst the Greeks, Romans, Egyptians, East Africans, Chinese and the Middle East grew considerably, a high quality high carbon iron and steel imported from Tamil people of the dynasty Chera.[21] Metal production sites in Sri Lanka utilized these novel techniques using unique wind furnaces driven by the monsoon winds, capable of producing high-carbon steel, as well as imported artefacts of ancient iron and steel from Kodumanal. Large-scale Wootz steel production in Tamilakam using crucibles they invented and carbon sources such as the plant Avāram occurred by the sixth century BC, the pioneering precursor to modern steel production and metallurgy.[22][23]

Steel was produced in large quantities in Sparta around 650 BC.[24][25]

The Chinese of the Warring States period (403–221 BC) had quench-hardened steel,[26] while Chinese of the Han dynasty (202 BC – 220 AD) created steel by melting together wrought iron with cast iron, gaining an ultimate product of a carbon-intermediate steel by the 1st century AD.[27][28] The Haya people of East Africa invented a type of furnace they used to make carbon steel at 1,802 °C (3,276 °F) nearly 2,000 years ago. East African steel has been suggested by Richard Hooker to date back to 1400 BC.[29][30]

Wootz, also known as Damascus steel, is famous for its durability and ability to hold an edge. It was originally created from a number of different materials including various trace elements, apparently ultimately from the writings of Zosimos of Panopolis. However, the steel was an old technology in India when King Porus presented a steel sword to the Emperor Alexander in 326 BC.[citation needed] It was essentially a complicated alloy with iron as its main component. Recent studies have suggested that carbon nanotubes were included in its structure, which might explain some of its legendary qualities, though given the technology of that time, such qualities were produced by chance rather than by design.[44] Natural wind was used where the soil containing iron was heated by the use of wood. The ancient Sinhalese managed to extract a ton of steel for every 2 tons of soil,[41] a remarkable feat at the time. One such furnace was found in Samanalawewa and archaeologists were able to produce steel as the ancients did.[41][45]

Crucible steel, formed by slowly heating and cooling pure iron and carbon (typically in the form of charcoal) in a crucible, was produced in Merv by the 9th to 10th century AD.[33] In the 11th century, there is evidence of the production of steel in Song China using two techniques: a "berganesque" method that produced inferior, inhomogeneous, steel, and a precursor to the modern Bessemer process that used partial decarbonization via repeated forging under a cold blast.[46]

Since the 17th century the first step in European steel production has been the smelting of iron ore into pig iron in a blast furnace.[47] Originally employing charcoal, modern methods use coke, which has proven more economical.[48][49][50]

In these processes pig iron was "fined" in a finery forge to produce bar iron (wrought iron), which was then used in steel-making.[47]

The production of steel by the cementation process was described in a treatise published in Prague in 1574 and was in use in Nuremberg from 1601. A similar process for case hardening armour and files was described in a book published in Naples in 1589. The process was introduced to England in about 1614 and used to produce such steel by Sir Basil Brooke at Coalbrookdale during the 1610s.[51]

The raw material for this process were bars of wrought iron. During the 17th century it was realized that the best steel came from oregrounds iron of a region north of Stockholm, Sweden. This was still the usual raw material source in the 19th century, almost as long as the process was used.[52][53]

Crucible steel is steel that has been melted in a crucible rather than having been forged, with the result that it is more homogeneous. Most previous furnaces could not reach high enough temperatures to melt the steel. The early modern crucible steel industry resulted from the invention of Benjamin Huntsman in the 1740s. Blister steel (made as above) was melted in a crucible or in a furnace, and cast (usually) into ingots.[53][54]

The modern era in steelmaking began with the introduction of Henry Bessemer's Bessemer process in 1855, the raw material for which was pig iron.[55] His method let him produce steel in large quantities cheaply, thus mild steel came to be used for most purposes for which wrought iron was formerly used.[56] The Gilchrist-Thomas process (or basic Bessemer process) was an improvement to the Bessemer process, made by lining the converter with a basic material to remove phosphorus.

Another 19th-century steelmaking process was the Siemens-Martin process, which complemented the Bessemer process.[53] It consisted of co-melting bar iron (or steel scrap) with pig iron.

These methods of steel production were rendered obsolete by the Linz-Donawitz process of basic oxygen steelmaking (BOS), developed in the 1950s, and other oxygen steel making methods. Basic oxygen steelmaking is superior to previous steelmaking methods because the oxygen pumped into the furnace limits impurities that previously had entered from the air used.[57] Today, electric arc furnaces (EAF) are a common method of reprocessing scrap metal to create new steel. They can also be used for converting pig iron to steel, but they use a lot of electrical energy (about 440 kWh per metric ton), and are thus generally only economical when there is a plentiful supply of cheap electricity.[58]

It is common today to talk about "the iron and steel industry" as if it were a single entity, but historically they were separate products. The steel industry is often considered an indicator of economic progress, because of the critical role played by steel in infrastructural and overall economic development.[59]

In 1980, there were more than 500,000 U.S. steelworkers. By 2000, the number of steelworkers fell to 224,000.[60]

The world steel industry peaked in 2007. That year, ThyssenKrupp spent $12 billion to build the two most modern mills in the world, in Calvert, Alabama and Sepetiba, Rio de Janeiro, Brazil. The worldwide Great Recession starting in 2008, however, sharply lowered demand and new construction, and so prices fell. ThyssenKrupp lost $11 billion on its two new plants, which sold steel below the cost of production. Finally in 2013, ThyssenKrupp offered the plants for sale at under $4 billion.[64]

Steel is one of the world's most-recycled materials, with a recycling rate of over 60% globally;[65] in the United States alone, over 82,000,000 metric tons (81,000,000 long tons) was recycled in the year 2008, for an overall recycling rate of 83%.[66]

Modern steels are made with varying combinations of alloy metals to fulfill many purposes.[3]Carbon steel, composed simply of iron and carbon, accounts for 90% of steel production.[1]Low alloy steel is alloyed with other elements, usually molybdenum, manganese, chromium, or nickel, in amounts of up to 10% by weight to improve the hardenability of thick sections.[1]High strength low alloy steel has small additions (usually < 2% by weight) of other elements, typically 1.5% manganese, to provide additional strength for a modest price increase.[67]

Recent Corporate Average Fuel Economy (CAFE) regulations have given rise to a new variety of steel known as Advanced High Strength Steel (AHSS). This material is both strong and ductile so that vehicle structures can maintain their current safety levels while using less material. There are several commercially available grades of AHSS, such as dual-phase steel, which is heat treated to contain both a ferritic and martensitic microstructure to produce a formable, high strength steel.[68] Transformation Induced Plasticity (TRIP) steel involves special alloying and heat treatments to stabilize amounts of austenite at room temperature in normally austenite-free low-alloy ferritic steels. By applying strain, the austenite undergoes a phase transition to martensite without the addition of heat.[69] Twinning Induced Plasticity (TWIP) steel uses a specific type of strain to increase the effectiveness of work hardening on the alloy.[70]

Carbon Steels are often galvanized, through hot-dip or electroplating in zinc for protection against rust.[71]

Stainless steels contain a minimum of 11% chromium, often combined with nickel, to resist corrosion. Some stainless steels, such as the ferritic stainless steels are magnetic, while others, such as the austenitic, are nonmagnetic.[72] Corrosion-resistant steels are abbreviated as CRES.

Some more modern steels include tool steels, which are alloyed with large amounts of tungsten and cobalt or other elements to maximize solution hardening. This also allows the use of precipitation hardening and improves the alloy's temperature resistance.[1] Tool steel is generally used in axes, drills, and other devices that need a sharp, long-lasting cutting edge. Other special-purpose alloys include weathering steels such as Cor-ten, which weather by acquiring a stable, rusted surface, and so can be used un-painted.[73]Maraging steel is alloyed with nickel and other elements, but unlike most steel contains little carbon 0.01%). This creates a very strong but still malleable steel.[74]

Eglin steel uses a combination of over a dozen different elements in varying amounts to create a relatively low-cost steel for use in bunker buster weapons. Hadfield steel (after Sir Robert Hadfield) or manganese steel contains 12–14% manganese which when abraded strain hardens to form an incredibly hard skin which resists wearing. Examples include tank tracks, bulldozer blade edges and cutting blades on the jaws of life.[75]

In 2015 a breakthrough in creating a strong light aluminium steel alloy which might be suitable in applications such as aircraft was announced by researchers at Pohang University of Science and Technology. Adding small amounts of nickel was found to result in precipitation as nano particles of brittle B2 intermetallic compounds which had previously resulted in weakness. The result was a cheap strong light steel alloy which is slated for trial production at industrial scale by POSCO, a Korean steelmaker.[76][77]

Iron and steel are used widely in the construction of roads, railways, other infrastructure, appliances, and buildings. Most large modern structures, such as stadiums and skyscrapers, bridges, and airports, are supported by a steel skeleton. Even those with a concrete structure employ steel for reinforcing. In addition, it sees widespread use in major appliances and cars. Despite growth in usage of aluminium, it is still the main material for car bodies. Steel is used in a variety of other construction materials, such as bolts, nails, and screws and other household products and cooking utensils. [80]

Before the introduction of the Bessemer process and other modern production techniques, steel was expensive and was only used where no cheaper alternative existed, particularly for the cutting edge of knives, razors, swords, and other items where a hard, sharp edge was needed. It was also used for springs, including those used in clocks and watches.[53]

With the advent of speedier and thriftier production methods, steel has become easier to obtain and much cheaper. It has replaced wrought iron for a multitude of purposes. However, the availability of plastics in the latter part of the 20th century allowed these materials to replace steel in some applications due to their lower fabrication cost and weight.[81]Carbon fiber is replacing steel in some cost insensitive applications such as aircraft, sports equipment and high end automobiles.

^Sharada Srinivasan (1994). [file:///C:/Users/user/Downloads/101-254-1-PB%20(1).pdf Wootz crucible steel: a newly discovered production site in South India]. Papers from the Institute of Archaeology 5(1994) 49-59